Control transformers are generally isolation transformers that provide a high degree of secondary voltage stability during a brief period of overload current (also known as “inrush current”), as may be the case when relay contacts close to energize an inductive load such as a coil of an electromechanical relay, or a motor, among many other situations. In an industrial control circuit application, a control transformer is a relatively small product where overall dimensions are very critical as they may be part of a programmable logic controller, or motor control center for example, which have limited space for the control transformer. A control transformer has a primary winding connected to a power source and one or more secondary windings connected to a load. The end of the windings, or winding taps, are terminated into terminals or terminal blocks where a user makes their connections.
To protect the transformer from any fault arising on the load side, various options are available. For example, one option is that secondary fuse(s) are used between secondary transformer terminals and the load circuit. In the event of fault, one or more secondary fuse blows and isolates the load from control transformer; however, in this case the transformer remains energized. In another option, fuses are used between the power source and the primary terminals of the control transformer. In the event of a fault in the load circuit or in the transformer, the primary fuse(s) blow; thus, the control transformer is disconnected from the source of power. In yet another option, fuses are used both on the primary and secondary side of the transformer. These fuses can be mounted on the control transformer or at another location of the electrical system which is associated with the control transformer, such as a circuit board.
Additionally, and in a single secondary winding arrangement, two wires come to secondary terminals from the load. Fuses can be installed on one wire or both, but connecting fuses on both wire gives complete load isolation from the power source through the control transformer. If there are more than one isolated secondary winding or multi-tapped windings, and if a user desires complete load isolation, then at least one fuse is required for each output to disrupt current to each load during a fault condition.
FIGS. 1A and 1B illustrate prior art examples where one or more fuses can be electrically connected to either of primary or secondary taps, and where the fuse holders are mounted on the transformer core or terminal board. External connection is made between the transformer winding and fuse holders. For example, and as can be seen in the prior art arrangements shown in FIGS. 1A and 1B, transformers 20 and 22 include a mounting plate 24, fuse holder 26, fuse clips 28, interconnecting wires 30 and their corresponding connectors 32, and fasteners 34. In the case of transformer 20, plate 24 and fuse holder 26 are mounted to core 36; whereas in the case of transformer 22, plate 24 and fuse holder 26 are mounted to terminal boards 38, which have at least some terminals connected to the primary winding taps or the secondary winding taps. The fuses are not shown in FIGS. 1A and 1B, but are held by fuse holders 26, and can typically be a glass cartridge type fuse, plug type fuse, or other types. However, there are several disadvantages to such an approach.
Firstly, the many different components add material and labor manufacturing costs to the transformer. Additionally, the multiple electrical connections, such as between connectors 32 and terminal boards 38, connectors 32 and wires 30, and connectors 32 and clips 28, reduce the reliability of the system by introducing many potential single point failure mechanisms where a loose, corroded, or otherwise contaminated or flawed connection can cause a malfunction in the electrical system in which transformers 20 and 22 are used.
Fusing arrangements are known in which a single fuse is mounted directly to the transformer terminal boards via fuse clips; however, such arrangements are limited, because of the size of the fuse relative to the spacing of the terminals on the transformer terminal board, to a single fuse on the primary side and/or the secondary side, which does not allow for complete isolation of either the power source or load.
What is needed in the art is a method and apparatus for directly mounting fuses to transformer terminals, and which can accommodate one or more fuses on either the primary or secondary side of the transformer.